The invention relates to an ultra high strength multiphase steel with dual-bainite-martensite- or complex-phase microstructure with improved properties during production and processing, in particular for the vehicle lightweight construction. The invention also relates to a method for producing steel strips made of such a steel.
The hotly contested automobile market forces manufacturers to constantly seek solutions to lower the fleet consumption while at the same time maintaining a highest possible comfort and occupant protection. Hereby weight saving of all vehicle components plays an important role but also a favorable behavior of the individual components in case of a high static or dynamic stress during use and also in case of a crash. Suppliers for starting material seek to account for this requirement by providing ultra-high-strength steel with thin sheet thickness to reduce the weight of the vehicle components while at the same time maintaining the same or even improved component properties.
Beside the required weight reduction these newly developed steels have to meet the high material demands regarding proof strength, tensile strength and elongation at break, as well as component demands regarding corner crack resistance, energy absorption and also have to have defined hardenings via the work-hardening effect and the bake-hardening effect. In addition good processability has to be ensured. This applies to the processes at the vehicle manufacturer (for example forming, welding or varnishing) as well as to the manufacturing processes at the supplier of starting material (for example surface treatment by metallic or organic coating).
The property combination demanded of the steel material in its consequence represents a component specific compromise of individual properties. In vehicle construction dual-phase steels are thus increasingly used which consist of a ferritic basic structure in which a martensitic second phase and possibly a further phase with bainite and residual austenite is integrated. The characteristic processing properties of the dual-phase steels such as a very low yield ultimate ratio at a very high tensile strength, strong strain hardening and good cold formability are well known.
Increasingly multiphase steels are also used in the automobile construction such as complex-phase steels, ferritic-bainitic steels, bainitic steels and also martensitic steels, which have different microstructure compositions.
Complex steels in hot rolled or cold rolled configuration are steels which have small proportions of martensite, residual austenite and/or perlite in a ferritic/bainitic basic structure, wherein as a result of a delayed recrystallization or precipitation of micro-alloy elements an extreme grain fineness is established.
Ferrite-bainitic steels in hot rolled configuration are steels, which contain bainite or work-hardened bainite in a matrix of ferrite and/or work-hardened ferrite. The work hardening of the matrix is caused by a high dislocation density, by grain refinement and the precipitation of micro-alloy elements.
Bainitic steels in hot rolled or cold rolled configuration are steels which are characterized by a very high yield strength and tensile strength at a sufficiently high expansion for cold forming processes. Due to the chemical composition a good weldability is given. The microstructure typically consists of bainite and ferrite. In some cases small proportions of other phases such as martensite and residual austenite can be contained.
Martensitic steels in hot rolled configuration are steels which as a result of thermo-mechanical rolling contain small proportions of ferrite and/or bainite in the basic structure of martensite. The steel type is characterized by a very high yield strength and tensile strength at a sufficiently high expansion for cold forming processes. Within the group of multiphase steels the martensitic steels have the highest tensile strength values.
These steels are also currently used in structural components, chassis and crash relevant components as well as flexibly cold rolled strips. This Tailor Rolled Blank lightweight construction technology (TRB®) enables a significant weight reduction as a result of the load adjusted selection of sheet thickness over the length of the component.
In the case of strongly varying sheet thicknesses, the production of TRB®s with multiphase microstructure is possible with todays known alloys and the available continuous annealing systems only with limitations such as regarding the heat treatment prior to the cold rolling. In regions of different sheet thickness no homogenous multiphase microstructure can be established in cold rolled and hot rolled steel strips due to the temperature difference in the conventional process windows.
For economic reasons cold rolled steel strips are usually subjected to recrystallizing annealing in the continuous annealing process to generate well formable steel sheet. Depending on the alloy composition and the strip cross section, the process parameters such as throughput speed, annealing temperature and cooling rate, are adjusted corresponding to the mechanical-technological properties by way of the microstructure required therefore.
For establishing the dual-phase microstructure the hot strip or cold strip is heated in the continuous annealing furnace to such a temperature that the required microstructure forms during the cooling. The same applies for configuring a steel with complex phase microstructure, martensitic, ferritic-bainitic and also purely bainitic microstructure.
When high demands on corrosion protection require the surface of the hot or cold strip to be hot dip galvanized, the annealing is usually carried out in a continuous hot dip galvanizing system in which the heat treatment or annealing and the downstream galvanizing occur in a continuous process.
Also in case of the hot strip, depending in each case on the alloying concept, the demanded microstructure is only established in the annealing in the continuous furnace in order to realize the demanded mechanical properties.
The continuous annealing of hot rolled or cold rolled steel strip for example with the alloy concepts for ultra-high-strength dual-phase steels with minimal tensile strengths of about 950 MPa known from EP 2018 282 A1 and EP 2031 081 A1, involve the problem that only a small process window is available for the annealing parameters. As a result adjustments of the process parameters are already required at minimal cross sectional changes (thickness, width) to achieve uniform mechanical properties.
In the case of widened process windows the required strip properties can also be achieved at same process parameters also in the case of greater cross sectional changes of the strips to be annealed.
Besides flexibly rolled blanks with different sheet thicknesses over the strip width this applies in particular also to strips with different thickness and/or different width which have to be annealed subsequent to each other.
Especially in the case of different thicknesses in the transition region of one strip to another, a homogenous temperature distribution is difficult to achieve. In the case of alloy compositions with too small process windows this can lead to the fact that for example the thinner strip is either moved through the furnace too slowly thereby lowering productivity, or that the thicker strip is moved though the furnace too fast and the required annealing temperature for achieving the desired microstructure is not reached. The result of this is more waste with the corresponding non-conformity costs.
The deciding process parameter is thus the adjustment of the speed in the continuous annealing because the phase transformation is temperature and time dependent. Thus, the less sensitive the steel is regarding the uniformity of the mechanical properties when temperature and time course change during the continuous annealing, the greater is the process window.
The problem of a too narrow process window is especially pronounced in the annealing treatment when stress-optimized components made of hot or cold strip are to be produced, which have sheet thicknesses that vary across the strip length and strip width (for example as a result of flexible rolling).
A method for producing a steel strip with different thickness across the strip length is for example described in DE 100 37 867 A1.
When using the known alloy concepts for the group of the multiphase steels, the narrow process window makes it already difficult during the continuous annealing of strips with different thickness to establish uniform mechanical properties over the entire length of the strip. Complex-phase steels in addition have an even narrower process window than dual-phase steels.
In the case of flexibly rolled cold strip made of multiphase steels of known composition, the too narrow process window either causes the regions with lower sheet thickness to have excessive strengths resulting from excessive martensite proportions due to the transformation processes during the cooling, or the regions with greater sheet thickness achieve insufficient strengths as a result of insufficient martensite proportions. Homogenous mechanical-technological properties across the strip length or width can practically not be achieved with the known alloy concepts in the continuous annealing.
The goal to achieve the resulting mechanical-technological properties in a narrow region across the strip width and strip length by the controlled adjustment of the volume proportions of the microstructure phases has highest priority and is therefore only possible by a widened process window. The known alloy concepts for multiphase steels are characterized by a too narrow process window and are therefore not suited for solving the present problem, in particular in the case of flexibly rolled strips. With the known alloy concepts to date only steels of a strength class with defined cross sectional regions (sheet thickness and strip width) can be produced so that different alloy concepts are required for different strength classes or cross sectional ranges.
The state of the art is to increase the strength by increasing the amount of carbon and/or silicone and/or manganese and via the microstructure adjustment and the solid solution strengthening (solid solution hardening).
However, as a result of increasing the amount of the aforementioned elements, the material processing properties increasingly worsen for example during welding, forming and hot dip coating.
On the other hand, there is also a trend in the steel production to reduce the carbon and/or manganese content in order to achieve a better cold processability and better performance properties.
An example is the hole expansion test for describing and quantifying the edge crack behavior. At corresponding optimized grades the steel user expects higher values than in the standard material. However, increasingly the focus is also on the welding suitability characterized by the carbon equivalent.
The automobile industry increasingly demands steel grades which have to meet significantly different requirements regarding the yield strength depending on the application. This leads to steel developments with comparatively great yield strength at conventional tensile strength interval.
A low yield strength ratio (Re/Rm) is typical for a dual-phase steel and serves in particular for the formability in stretching and deep drawing processes.
A higher yield strength ratio (Re/Rm) as it is typical for complex-phase steels is also characterized by a resistance against edge cracks. This can be attributed to the smaller differences in the strengths of the individual microstructure components, which has a positive effect on a homogenous deformation in the region of the cutting edge.
The analytical landscape for achieving multiphase steels with minimal strengths of 950 MPa has become more diverse and shows very broad alloy ranges regarding the strength-promoting elements carbon, manganese, phosphorous, aluminum and chromium and/or molybdenum as well as regarding the addition of micro-alloys individually or in combination and regarding the material characterizing properties.
The spectrum regarding dimensions is broad and lies in the thickness range of 0.50 to 3.00 mm, wherein the range between 0.8 to 2.1 mm is relevant. Predominantly slit strip dimensions and also wide strips up to 1700 mm are used.